Problem: What is the greatest common factor of $21$ and $3$ ? Another way to say this is: $\operatorname{gcf}(21, 3) = {?}$
Explanation: The greatest common factor is the largest number that is a factor of both $21$ and $3$ The factors of $21$ are $1$ $3$ $7$ , and $21$ The factors of $3$ are $1$ and $3$ Thus, the greatest common factor of $21$ and $3$ is $3$. $\operatorname{gcf}(21, 3) = 3$